The present invention relates to a method and apparatus for terminal connection for highly tensioned tubular conduits. More particularly the present invention relates to a method and system for flexibly connecting pressurized, highly tensioned tubular elements across a joint that must accommodate relative motion between its joined ends. The present invention will be seen to have particularly beneficial application to risers extending from subsea facilities to compliant structures in the development of offshore oil and gas reserves.
Traditional bottom-founded platforms having fixed or rigid tower structures have been taken to their logical depth limits in the development of offshore oil and gas reserves. Economic considerations suggest that alternatives to this traditional technology be used in the development of deepwater prospects and various xe2x80x9ccompliant structuresxe2x80x9d have been proposed or developed. Among these alternatives are tension leg platforms, compliant towers, articulated towers, floating production facilities, tension leg well jackets, and spar structures. Common to these alternatives is the fact that surface facilities supported by the platform are in relative motion with subsea facilities. Thus, the high pressure conduits, e.g., import, export, and production risers, that connect the subsea and surface facilities in the transport of hydrocarbons must accommodate constant relative motion. Further, these conduits may well be 1000 meters or more long and surface support may highly tension the conduits at their platform termination.
Flexible joints have been developed to accommodate this relative motion. However, these components combine exotic materials in complex manufacturing operations and prove relatively expensive to acquire. Further, the dynamic service life of flexible joints may prove less than that of the risers or of the platform. This may require additional maintenance for the riser, at least in replacement of these components. Such flexible joint applications are illustrated in U.S. Pat. No. 5,269,629 issued to Langner on Dec. 14, 1993 and U.S. Pat. No. 5,447392 issued to Marshall on Sep. 5, 1995.
Titanium stress joints theoretically have the necessary fatigue life, but titanium is too expensive for the overall riser application and proves susceptible to service problems when placed in seawater in combination with steel riser sections and other adjacent subsea and platform components. A titanium alloy stress joint subject to these problems is illustrated by U.S. Pat. 4,188,156 issued to Fisher et al on Feb. 12, 1980.
Thus, there remains a need for a simple termination or joint to accommodate relative motion at terminations in conduits for high pressure, high load applications.
Toward providing these and other advantages, the present invention is a monolithic isolation stress joint having a first conduit element, a first insulating joint assembly, and a stress joint connected to the first conduit element through the first insulating joint assembly. The stress joint is formed of a material which has advantageous elastic flexure characteristics but which is electrochemically active with respect to the first conduit element from which it is electrically isolated by the first insulating joint assembly. A second conduit element is connected to the stress joint through a second insulating joint assembly, the second conduit element being formed of a material which is electrochemically active with respect to the stress joint and which is electrically isolated therefrom with the second insulating joint.